This invention relates to an improved fuel injector which effectively controls the flow rate of fuel injected into the combustion chamber of an engine.
In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-needle injector which includes a needle assembly having a spring-biased needle valve element positioned adjacent the needle orifices for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The needle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The needle valve is positioned in a needle cavity and biased by a needle spring to block fuel flow through the needle orifices. In many fuel systems, when the pressure of the fuel within the needle cavity exceeds the biasing force of the needle spring, the needle valve element moves outwardly to allow fuel to pass through the needle orifices, thus marking the beginning of injection. In another type of system, such as disclosed in U.S. Pat. No. 5,676,114 to Tarr et al., the beginning of injection is controlled by a servo-controlled needle valve element. The assembly includes a control volume positioned adjacent an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve. U.S. Pat. No. 5,463,996 issued to Maley et al. discloses a similar servo-controlled needle valve injector.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate. As a result, many proposals have been made to provide injection rate control devices in closed needle fuel injector systems. One method of controlling the initial rate of fuel injection is to spill a portion of the fuel to be injected during the injection event. For example, U.S. Pat. No. 5,647,536 to Yen et al. discloses a closed needle injector which includes a spill circuit formed in the needle valve element for spilling injection fuel during the initial portion of an injection event to decrease the quantity of fuel injected during this initial period thus controlling the rate of fuel injection. A subsequent unrestricted injection flow rate is achieved when the needle valve moves into a position blocking the spill flow causing a dramatic increase in the fuel pressure in the needle cavity. However, the needle valve is not servo-controlled and, thus, this needle assembly does not include a control volume for controlling the opening and closing of the needle valve. Moreover, the rate shaping needle assembly does not permit the rate to be selectively varied.
Other rate shaping systems decrease rate of fuel flow during the initial portion of the injection event by, for example, throttling the fuel to the needle orifices. Although these systems create injection rate shaping, the spilling and throttling of fuel during the initial period of injection achieves a reduced injection flow rate by reducing the injection pressure adjacent the needle orifices. The decrease in injection pressure may disadvantageously result in decreased atomization of the fuel spray by the needle orifices, thus adversely affecting fuel economy and increasing emissions.
Another manner of optimizing combustion is to create pilot and/or post injection events. Most current diesel injectors include fixed needle orifice areas sized to provide optimum injection duration at rated speed and load with the highest allowable injection pressure. However, in order to optimize combustion, pilot and post injection events must include extremely small quantities of fuel at high injection pressures. With a fixed spray orifice size, this results in an extremely short event that is difficult to control. To compensate, the needle opening velocity may be reduced so that the fuel flow is throttled before the spray orifices during the pilot and post injection events. However, needle velocity is not easily controllable from injector to injector, while throttling wastes fuel energy and does not provide optimum combustion performance. At low speed and light load, it is also desirable to have small spray orifices to increase injection duration without lowering injection pressure.
Another fuel injector design providing some limited control over fuel injection rate and quantity includes two needle valve elements for controlling the flow of fuel through respective sets of injection orifices. For example, U.S. Pat. No. 5,458,292 to Hapeman discloses a fuel injector with inner and outer injector needle valves biased to close respective sets of spray holes and operable to open at different fuel pressures. The inner needle valve is reciprocally mounted in a central bore formed in the outer needle valve. However, the opening of each needle valve is controlled solely by injection fuel pressure acting on the needle valve in the opening direction such that the valves necessarily open when the injection fuel pressure reaches a predetermined level. Consequently, the overall and relative timing of opening of the valves, and the rate of opening of the valves, cannot be controlled independently. Moreover, the valve opening timing and rate is undesirably dependent on the injection fuel pressure.
U.K. Pat. Application No. 2266559 to Hlousek discloses a closed needle injector assembly including a hollow needle valve for cooperating with one valve seat formed on an injector body to provide a ma in injection through all the injector orifices and an inner valve needle reciprocally mounted in the hollow needle for creating a pre-injection through a few of the injector orifices. However, the valve seat allowing the inner valve needle to block the pre-injection flow is formed on the hollow valve member and the inner valve needle is biased outwardly away from the injector orifices. This arrangement requires a third valve seat for cooperation with the inner valve element when in a pre-injection open position to prevent flow through all of the injector orifices, resulting in an unnecessarily complex and expensive assembly. Also, this assembly is designed for use with two different sources of fuel requiring additional delivery passages in the injector. In addition, like Hapeman, this design requires the timing and rate of opening of at least one of the needle valves to be controlled by fuel injection pressure thereby limiting injection control.
U.S. Pat. No. 5,199,398 to Nylund discloses a fuel injection valve arrangement for injecting two different types of fuels into an engine which includes inner and outer poppet type needle valves. During each injection event, the inner needle valve opens a first set of orifices to provide a preinjection and the outer needle valve opens a second set of orifices to provide a subsequent main injection. The outer poppet valve is a cylindrical sleeve positioned around a stationary valve housing containing the inner poppet valve.
U.S. Pat. No. 5,899,389 to Pataki et al. discloses a fuel injector assembly including two biased valve elements controlling respective orifices for sequential operation during an injection event. A single control volume may be provided at the outer ends of the elements for receiving biasing fluid to create biasing forces on the elements for opposing the fuel pressure opening forces. However, only one control volume is used thereby preventing independent selective control of the elements.
Although some systems discussed hereinabove create different stages of injection, further improvement is desirable. Therefore, there is need for a servo-controlled fuel injector for providing enhanced selective control over injection timing and duration and variable control of injection rate shaping.
It is an object of the present invention, therefore, to overcome the disadvantages of the prior art and to provide a fuel injector which is capable of effectively and predictably controlling the rate of fuel injection.
It is another object of the present invention to provide a servo-controlled injector capable of effectively controlling the flow rate of fuel injected during each injection event so as to minimize emissions.
It is another object of the present invention to provide a servo-controlled injector assembly capable of shaping the rate of fuel injection which is also simple and inexpensive to manufacture.
It is yet another object of the present invention to provide an injector capable of effectively slowing down the rate of fuel injection during the initial portion of an injection event while subsequently increasing the rate of injection to rapidly achieve a high injection rate.
It is a further object of the present invention to provide an injector for use in a variety of fuel systems, including common rail system, accumulator pump systems and pump-line-needle fuel systems, which effectively controls the rate of injection at each cylinder location.
Still another object of the present invention is to provide a rate shaping injector which is capable of selectively creating numerous injection rate shapes to optimize emissions and fuel economy.
Yet another object of the present invention is to provide an injector which offers maximum flexibility in controlling fuel injection quantities during pilot and post injections while permitting injection rate shaping.
Another object of the present invention is to provide an injector which relaxes the need for a fast event, single actuator.
These and other objects of the present invention are achieved by providing a closed nozzle injector assembly for injecting fuel at high pressure into the combustion chamber of an engine, comprising an injector body containing an injector cavity and a plurality of injector orifices communicating with one end of the injector cavity to discharge fuel into the combustion chamber wherein the plurality of injector orifices include a first set of orifices and a second set of orifices and the injector body includes a fuel transfer circuit for transferring supply fuel to the plurality of injector orifices. The injector also includes a first needle valve element positioned in the injector cavity for controlling fuel flow through a first set of injector orifices and a first valve seat formed on the injector body. A first needle valve element is movable from a closed position against the first valve seat blocking flow through the first set of injector orifices to an open position permitting flow through the first set of injector orifices. A second needle valve element is also provided and positioned in the injector cavity for controlling fuel flow through the second set of injector orifices, and a second valve seat is provided and formed on the injector body. The second valve element is movable from a closed position against the second valve seat blocking flow through the second set of injector orifices to an open position permitting flow through the second set of injector orifices. A first control volume is positioned adjacent an outer end of the first needle valve element while a first control volume charge circuit is used to supply fuel from the fuel transfer circuit to a first control volume. A first drain circuit is provided for draining fuel from the first control volume to a low pressure drain while a first injection control valve is positioned along a first drain circuit for controlling the flow of fuel through the first drain circuit to control movement of the first needle valve element between the opened and closed positions. A second control volume is also provided and positioned adjacent an outer end of the second needle valve element while a second control volume charge circuit supplies fuel from the fuel transfer circuit to the second control volume. Likewise, a second drain circuit is provided for draining fuel from the second control volume to a low pressure drain while a second injection control valve is positioned along the second drain circuit for controlling the flow of fuel through the second drain circuit to control movement of the second needle valve element between the opened and closed positions.
The second needle valve element may be telescopingly received within a cavity formed in the first needle valve element to form a sliding fit with an inner surface of the first needle valve element. The first and second injection control valves may each include an actuator and a reciprocally mounted, selectively movable control valve member. The actuator may include a solenoid assembly. The injector may further include a first biasing spring for biasing the first needle valve element toward a closed position and a second biasing spring for biasing the second needle valve element toward the closed position wherein both the first and second needle valve elements extend through inner radial extents of both the first and second biasing springs. The first and: second biasing springs may be positioned in nonoverlapping serial relationship along a longitudinal axis. The injector may further include a second spring seat assembly for abutment by the second biasing spring including a transverse extension engaging the second needle valve element and extending from the second needle valve element through an aperture formed in the first needle valve element. The aperture may be elongated in shape and the second spring seat assembly may further include an annular seat connected to the transverse extension for abutment by the second biasing spring. The actuators for the first and second needle valve elements may be positioned adjacent one another in side-by-side relationship with respective axes of reciprocation of the control valve members positioned in parallel. The first control volume may be positioned along a longitudinal axis of the injector body between the injector orifices and the second control volume. Also, the first control volume may be positioned along a longitudinal axis of the injector body between the second control volume and the first and second biasing spring.